The present invention relates to vitamin D compounds, and more particularly, to a method of presenting the 1α-OH of vitamin D compounds in an axial orientation and the compounds made thereby.
The two diastereomeric forms of monosubstituted cyclohexanes (Scheme I) are differently populated, the equilibrium constant K being given by the equationΔG°=−RT1n Kwhere K=[equatorial conformer]/[axial conformer]. ΔG° (usually negative) is the difference of free energy between the equatorial and axial conformers and −ΔG° is known as conformational free energy of the substituent R [defined as it's A value, Winstein et al., J. Am. Chem. Soc. 77, 5562 (1955)]. Thus, the greater the A value of the substituent R, the greater a driving force to adopt the R-equatorial form. A value can be, therefore, considered as
destabilization energy imparted to a monosubstituted six-membered chair by an axial substituent. Thus, for example, the A value of methyl substituent equals ca. 1.7 kcal/mol [Hirsch, Top. Stereochem. 1, 199 (1967)] that corresponds to 95% of population of equatorial conformer of methylcyclohexane at room temperature. The conformational free energies of substituents in cyclohexanes under ideal conditions are expected to be additive. It is usually assumed that all conformational effects are additive, i.e. various destabilizing interactions identified within a six-membered ring system operate independently of each other. In di-, tri- and polysubstituted cyclohexanes mutual interactions among the substituents have to be considered. Such interactions can destabilize one chair conformation raising its energy to favor an alternate inverted chair form, or even favor some other, distorted (rigid or flexible) cyclohexane geometries. The most important interactions that influence the equilibrium between the respective chair conformations include interaction of a pair of substituents in 1,2-trans-diequatorial and 1,3-cis-diaxial relationship. Thus, total destabilization energy (ED) can be described as a sum of the substituents' A values, representing monoaxial interactions, G values for 1,2-diequatorial interactions and U values for 1,3-diaxial interactions [Corey, et al., J. Org. Chem. 45, 765 (1980)].ED=Σ(A+G+U)
In the case of alkylidenecyclohexanes, additional interactions are involved, especially in 2-substituted derivatives. The most important interaction (designated A1,3-strain, Johnson, Chem. Rev. 68, 375 (1968)] exists in the allylic segment between equatorial R1 and substituent R2 of the exomethylene unit (Scheme II). When both
R1 and R2 are medium or large groups, the axial conformer is preferred over the equatorial (Malhotra et al., J. Am. Chem. Soc. 87, 5492 (1965)]. Thus, for example, when R1=R2=Me the difference in energy between both forms is approximately 4.5 kcal/mol, in favor of the axial conformer. In the case when R1=Me and R2=H, a 1:3 peri interaction exists which increases by ca. 1.25 kcal/mol the destabilization energy of the system (Duraisamy et al., J. Am. Chem. Soc. 105, 3264 (1983)].
Conformational behavior of vitamin D has attracted considerable attention over the past 25 years. It has been suggested long ago [Havinga, Experientia 29, 1181 (1973)] that vitamin D compounds can exist as a mixture of two rapidly equilibrating A-ring chair conformers. These two conformations were abbreviated as α- and β-forms (Scheme III). 1H NMR studies of vitamin D2 and D3 in chloroform solutions confirmed the existence of the dynamic equilibrium between the two chair forms [La Mar et al., J. Am. Chem. Soc. 96, 7317 (1974); Wing et al., J. Am. Chem. Soc. 97, 4980 (1975)] of these B-ring secosteroids. A similar conformational equilibrium has also been found for 25-hydroxyvitamin D3 (25-OH-D3), 1α-hydroxyvitamin D3 (1α-OH-D3) and the natural hormone 1α,25-dihydroxyvitamin D3 (1α,25-(OH)2D3) as well as some other A-ring substituted vitamin D derivatives [see for example Helmer et al., Arch. Biochem. Biophys. 241, 608 (1985); Sheves et al., J. Org. Chem. 42, 3597 (1977); Berman et al., J. Org. Chem. 42, 3325 (1977); Sheves et al., J. Chem. Soc. Chem. Commun. 643 (1975); Okamura et al., J. Org. Chem. 43, 574 (1978)]. In the α-chair conformer of vitamin D molecule, the hydroxy group is equatorial whereas in the β-chair conformer the hydroxy group is axially oriented.

NMR studies of various vitamin D compounds in solutions have also shown that the ratio of the respective A-ring conformers depends significantly on the solvent used [Helmer et al., Arch. Biochem. Biophys. 241, 608 (1985)]. Unfortunately, due to solubility problems, it is impossible to study these conformer populations in an aqueous medium. X-Ray diffraction studies of vitamin D2 and D3 confirmed that their A-rings also occur in the solid phase as an equimolar mixture of such extreme α- and β-chair conformations [Hull et al., Acta Cryst., Sect. B, 32, 2374 (1976); Trinh et al., J. Org. Chem. 41, 3476 (1976)]. Interestingly, 25-OH-D3 exists in the solid state exclusively in the α-form whereas the natural hormone 1α,25-(OH)2D3 in the A-ring β-form [Trinh et al., J. Chem. Soc., Perkin Trans. II, 393 (1977); Suwinska et al., Acta Cryst., Sect. B, 52, 550 (1996)]. X-Ray studies have also shown that the C(5)=C(6)-C(7)=C(8) diene part of the molecule is nearly planar, whereas the exocyclic C(10)=C(19) bond, because of steric strain, is twisted out of plane by about 55°. This exomethylene group is situated below the mean A-ring plane in the α-chair form and above it in the alternate β-chair form. In the case of vitamin D analogs substituted in the ring A with a 1α-hydroxy group, crucial for biological activity, the orientation of 1α-OH is axial in the a chair form and equatorial in the β-form (Scheme IV).

It has to be added that molecular mechanics calculations revealed that, similarly as in the case of the model 1,2-dimethylenecyclohexane ([Hofmann et al., J. Org. Chem. 55, 2151 (1990)], an existence of other than low-energy chair conformations of the ring A can be expected for D vitamins, namely, half-chair or twist forms [Mosquera et al., J. Mol. Struct. 168, 125, (1988); Hofer et al., Monatsh. fur Chemie, 124, 185 (1993)].
In 1974, it was proposed [Okamura et al., Proc. Natl. Acad. Sci. USA 71, 4194 (1974); Wing et al., Science 186, 939 (1974)] that calcium regulation ability of vitamin D is limited to the compounds that can assume a ring-A chair conformation in which the 1α-hydroxy group (or pseudo-1α-OH) occupies an equatorial orientation. Such conformation, according to this hypothesis, has the proper geometry for binding to the protein receptor, a step which is necessary to induce the biological response leading to the calcium transport and calcium mobilization in the body. However, recent results of biological testing of 1α,25-dihydroxy-10, 19-dihydroxyvitamin D3 compounds do not support the idea that the equatorially favored 1-hydroxyl would be the most biologically active. On the contrary, 1α,25-dihydroxy-10(S), 19-dihydrovitamin D3, the analog strongly biased toward the A-ring chair conformer possessing 1α-axial orientation, provided the greatest in vivo biological response and showed very significant activity on intestinal calcium transport. Moreover, more recent studies on 19-norvitamins, especially those substituted at C-2, demonstrate that pronounced biological activity is provided by compounds having an axial 1α-hydroxyl. Thus, it is believed that axial orientation of the 1α-hydroxyl group in the vitamin D molecule is of crucial importance for its biological activity and, the prediction of its biological response can be made by evaluation of the conformational equilibrium of the A-ring of the vitamin. It is believed that the more favored the axial position of 1α-hydroxyl is the greater biological response can be expected. A logical extension of this prediction is that the greatest activity can be predicted for such A-ring substitution of vitamin D molecules which:
1) constitute anancomeric system or other corresponding to at least 90% preponderating conformer possessing 1α-OH in axial position, even though the rate of A-ring inversion can remain facile—these analogs are characterized by conformationally free well-defined geometries of A rings and a significant energy advantage (at least 1.2 kcal/mole) for an axial 1α-OH conformer; or 2) constitute conformationally locked, rigid or distorted geometries in which the A ring is held in only one chair conformation, i.e. the one having an axial 1α-OH or, although it may deform considerably, it may not flip over to its conformationally inverted opposite form with equatorial orientation of 1α-OH.
Such structural constrains which prevent the cyclohexane ring from flipping but which can be accommodated by its chair geometry (Scheme V) include:
1) anchoring bonds (trans-fusion bonds to a ring of size seven or smaller),
2) flattening bonds (fusion bonds to a ring of size seven or smaller which contains a double bond exocyclic to the six-membered ring), and
3) bridged bonds (two contiguous ring bonds whose termini are joined by a bridge of five atoms or fewer).

It should be noted that the remaining substituents (or hydrogens) of flattening or anchoring bonds must assume an axial orientation with respect to the six-membered ring. In the case of (1,3)-bridging, the bridged bonds have to be axially disposed with respect to the six-membered ring.
Accordingly, the present invention provides a novel class of 1α-hydroxylated vitamin D compounds wherein the conformational equilibrium of the A-ring has, or has been altered or modified to favor a chair conformation that presents the 1α-hydroxyl in the axial orientation, and the A-ring is attached to the conventional 5,7-diene and C-D ring system having any known side chain attached at carbon 17 of the D-ring.
Structurally these novel analogs are characterized by the general formula I shown below:
where Y1 and Y2, which may be the same or different, are each selected from the group consisting of hydrogen and a hydroxy-protecting group; where Y3, Y4, Y5, Y6, Y7 and Y8, which may be the same or different, are each selected from the group consisting of hydrogen, a methyl group or substituted methyl group of the formula —CR1R2R3, an amino group or substituted amino group of the formula —NR1R2, a phosphino group or substituted phosphino group of the formula —PR1R2, an alkylsulfinyl group, an arylsulfinyl group, an alkylsulfonyl group, an arylsulfonyl group, and aryl, where R1, R2 and R3 are each independently selected from the group consisting of hydrogen, C1-5 alkyl, hydroxyalkyl, aminoalkyl, halogenalkyl, alkoxyalkyl, aryloxyalkyl, aryl, halogen, hydroxyl, protected hydroxy, alkoxyl, aryloxyl, acyl, an amino group, an alkyl substituted amino group, and an aryl substituted amino group, and where R1 and R2 taken together represent an oxo group or a group —(CH2)m— where m is an integer having a value of from 2 to 5; or Y3 and Y4 when taken together represent a methylene group; or Y7 and Y8 when taken together represent a methylene group; where Y2 and Y6, or Y2 and Y7, when taken together may represent the group —(CR1R2)n— where n is an integer having a value of from 1 to 4 and wherein any of the groups —CR1R2— may be replaced by an oxygen, sulfur or nitrogen atom; where Y5 and Y8, or Y5 and Y3, or Y3 and Y8, when taken together may represent the group —(CR1R2)r— where r is an integer having a value of from 1 to 5 and wherein any of the groups —CR1R2— may be replaced by an oxygen, sulfur or nitrogen atom; and where Y5 and Y6 when taken together represent the group ═CR4R5 where R4 and R5, which may be the same or different, are each selected from the group consisting of hydrogen and Y3, with the proviso that R4 and R5 cannot be a hydroxyl; and where R4 and Y2 when taken together may represent the group —(CR1R2)s— where s is an integer having a value of from 1 to 3; and where the group R represents any of the typical side chains known for vitamin D type compounds.
More specifically R can represent a saturated or unsaturated hydrocarbon radical of 1–35 carbons, that may be straight-chain, branched or cyclic and that may contain one or more additional substituents, such as hydroxy- or protected-hydroxy groups, fluoro, carbonyl, ester, epoxy, amino or other heteroatomic groups. Preferred side chains of this type are represented by the structure below
where the stereochemical center (corresponding to C-20 in steroid numbering) may have the R or S configuration, (i.e. either the natural configuration about carbon 20 or the 20-epi configuration), and where Z is selected from Y, —OY, —CH2OY, —C≡CY and —CH═CHY, where the double bond may have the cis or trans geometry, and where Y is selected from hydrogen, methyl, —COR10 and a radical of the structure:
where x and y, independently, represent the integers from 0 to 5, where R6 is selected from hydrogen, deuterium, hydroxy, protected hydroxy, fluoro, trifluoromethyl, and C1-5-alkyl, which may be straight chain or branched and, optionally, bear a hydroxy or protected-hydroxy substituent, and where each of R7, R8, and R9, independently, is selected from deuterium, deuteroalkyl, hydrogen, fluoro, trifluoromethyl and C1-5 alkyl, which may be straight-chain or branched, and optionally, bear a hydroxy or protected-hydroxy substituent, and where R6 and R7, taken together, represent an oxo group, or an alkylidene group, ═CR7R8, or the group —(CH2)p—, where p is an integer from 2 to 5, and where R8 and R9, taken together, represent an oxo group, or the group —(CH2)q—, where q is an integer from 2 to 5, and where R10 represents hydrogen, hydroxy, protected hydroxy, or C1-5 alkyl and wherein any of the CH-groups at positions 20, 22, or 23 in the side chain may be replaced by a nitrogen atom, or where any of the groups —CH(CH3)—, —CH(R3)—, or —CH(R2)— at positions 20, 22, and 23, respectively, may be replaced by an oxygen or sulfur atom.
The wavy line to the substituent at C-20 indicates that the carbon 20 may have either the R or S configuration.
Specific important examples of side chains with natural 20R-configuration are the structures represented by formulas (a), (b), (c), (d) and (e) below, i.e. the side chain as is occurs in 25-hydroxyvitamin D3 (a); vitamin D3 (b); 25-hydroxyvitamin D2 (c); vitamin D2 (d); and the C-24 epimer of 25-hydroxyvitamin D2 (e):

The above novel compounds wherein the 1α-OH group is presented in the axial orientation exhibit a desired, and highly advantageous, pattern of biological activity. These compounds are characterized by having greater biological activity, as compared to 1α,25(OH)2D3, in one or more of the three activities typically referred to as “calcemic” activities, i.e. intestinal calcium transport activity, bone mineralization activity and bone calcium mobilization activity, or in cell differentiation activity. Hence, these compounds may be highly specific in their calcemic activity. Their preferential calcemic activity suggests the in vivo administration of these compounds for the treatment of metabolic bone diseases where bone loss is a major concern. Because of their preferential calcemic activity on bone, one or more of these compounds may be preferred therapeutic agents for the treatment of diseases where bone formation is desired, such as osteoporosis, especially low bone turnover osteoporosis, steroid induced osteoporosis, senile osteoporosis or postmenopausal osteoporosis, as well as hypoparathroidism, osteomalacia and renal osteodystrophy. In addition, hypocalcemia as well as rickets, and vitamin D resistant rickets may be treated with one or more of the disclosed compounds. These compounds may also provide a method of treating female infertility in female mammals. The treatment may be transdermal, oral (in solid or liquid form) or parenteral. The compounds may be present in a composition in an amount from about 0.01 μg/day to about 100 μg/day, preferably about 0.1 μg/gm to about 50 μg/gm of the composition, and may be administered in dosages of from about 0.1 μg/day to about 50 μg/day.
The compounds of the invention are also especially suited for treatment and prophylaxis of human disorders which are characterized by an imbalance in the immune system, e.g. in autoimmune diseases, including multiple sclerosis, diabetes mellitus, host versus graft reaction, and rejection of transplants; and additionally for the treatment of inflammatory diseases, such as rheumatoid arthritis and asthma, as well as the improvement of bone fracture healing and improved bone grafts. Acne, alopecia, skin conditions such as dry skin (lack of dermal hydration), undue skin slackness (insufficient skin firmness), insufficient sebum secretion and wrinkles, and hypertension are other conditions which may be treated with one or more of the compounds of the invention.
The above compounds may also be characterized by high cell differentiation activity. Thus, these compounds may also provide therapeutic agents for the treatment of psoriasis and other skin disorders characterized by proliferation of undifferentiated skin cells, e.g. dermatitis, eczema, solar keratosis and the like, or as an anti-cancer agent, especially against leukemia, colon cancer, breast cancer and prostate cancer. The compounds may be present in a composition to treat disorders such as psoriasis in an amount from about 0.01 μg/gm to about 100 μg/gm of the composition, and may be administered topically, transdermally, orally (in solid or liquid form) or parenterally in dosages of from about 0.01 μg/day to about 100 μg/day.
This invention also provides a novel method of modifying or altering the structure of a 1α-hydroxylated vitamin D compound to increase its biological activity by altering the conformational equilibrium of the A-ring of the 1α-hydroxylated vitamin D compound to favor a chair conformation that presents the 1α-hydroxyl in the axial orientation. This is accomplished by either locking the chair conformation of the A-ring in a geometry having an axially orientated 1α-hydroxyl, or by the addition of one or more substituents to the A-ring which interact with other substituents in the molecule or on the A-ring to provide a driving force to the A-ring to adopt a chair conformation which presents the 1α-hydroxyl in the axial orientation.